Gay is a player that likes to create his own offense, which on the Bulls is sorely needed because other than Rose nobody can.

Memphis doesn't have a ton of offensive creators either, but given the make up of their team seems to be inside out Deng's ability to catch and shoot would likely be better utilized in Memphis. Also Deng is a better defender than Gay, which would give them a fantastic defensive duo on the wings to bother teams.

As for which player I think is better, tbh i'd call it about as close to a wash as any two players in the NBA. Both have great games, but they are different players with different roles.